As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which exhibit high energy density and voltage, long lifespan and low self-discharge rate are commercially available and widely used.
As positive electrode active materials for such lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure and the like, and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
In particular, lithium manganese-containing oxides such as LiMnO2, LiMn2O4, and the like, are advantageous in that they contain Mn, which is an abundant and environmentally friendly raw material. In addition, high-capacity lithium secondary batteries may be manufactured using the lithium manganese-containing oxides. As such, lithium manganese-containing oxides are receiving attention as a positive electrode active material of lithium secondary batteries.
However, when a lithium manganese-containing oxide is applied to a positive electrode, Mn3+ ions in the lithium manganese-containing oxide are decomposed into Mn2+ and Mn4+ as cycling of a lithium secondary battery proceeds, whereby a capacity of a positive electrode is considerably reduced. In particular, Mn2+ is dissolved in an electrolyte solution, decomposing an electrolyte solution while being deposited on a negative electrode having a lower potential. Accordingly, cycle characteristics are rapidly deteriorated.
Therefore, there is an urgent need for technology to resolve such problems.